1. Field of the Invention
This invention relates to a pipetting device comprising a housing, from the top end of which an actuating element protrudes, which is adapted to be depressed against spring action and is connected to a piston that is movable in a cylinder to move an air volume, which cylinder communicates with an opening of a tubular pipette connector, particularly a connector cone, for connecting the cylinder to an open-topped and open-bottomed pipette tip, wherein the actuating element is adapted to be rotated to adjust the pipettable volume.
2. Description of the Prior Art
Such pipetting devices are known. The invention is particularly based on a pipetting device which is disclosed in U.S. Pat. No. 4,041,764, the disclosure of which is incoporated herein by reference. That known device comprises special retaining means, which may consist of magnetic retaining means. That known device comprises in its lower portion a piston rod, which is movable within a cylinder for moving an air volume relative to the open bottom end of the housing in which the cylinder is terminated. By that movement of an air volume a liquid sample can be sucked into or dispensed from the pipette without contacting the piston
In that known design an adjustment of the pipettable volume is permitted by the provision of camlike stops, which are spaced around the periphery of the actuating element on different levels. By means of mechanical detent means provided on the actuating element, which may consist of an actuating knob, said stops may be adjusted to engage an abutment in order to set the device for a selected pipettable volume.
The known design may also comprise springs for holding cooperating parts together if magnetic means for that purpose are not provided.
In that known design the pipettable volume can be adjusted only in steps.
It is also known, e.g., from German Patent Publication No. 28 08 649 based on U.S. Pat. application Ser. No. 783,051 filed on Mar. 30, 1977 and now U.S. Pat. No. 4,098,125, to permit an infinite adjustment in that the piston rod extends in a tube, which is provided with external screw threads, on which a sleeve is screwed, which can be reciprocated in that the sleeve and the tube are rotated relative to each other so that the piston can also infinitely be adjusted. In that case a fixing must not be loosened before and tightened after each adjustment. For this reason that device is expensive and great care is required in its operation.
An adjustable pipette is known, which also comprises a screwable sleeve, which is coupled to a piston rod by a disklike enlarged portion provided at the bottom end of said piston rod and held in coupling engagement with the sleeve by spring action.
That known pipette is not reliable in operation, particularly because longitudinal grooves formed in the outside surface of the sleeve cooperate with a closing tip, which is urged against the sleeve by a leaf spring. As a result, a snap-action joint is provided, which does not permit a fine adjustment which generates a noise during an adjustment.
European Pat. No. 112,887 discloses a volume-controllable pipette, which has a detent mechanism, which has ball-shaped projections, which cooperate with corresponding recesses formed in a different element so that the volume can be adjusted. That device also permits an adjustment only in coarse steps, the adjustment results in a generation of noise and a fine adjustment is not permitted.
A known microliter pipette comprises a brake by which a volume setting can be fixed. But such brake can be overcome and it hinders the adjustment and in case of an insufficient pressure applied will not reliably fix the setting.
Those known devices which permit of a fine adjustment have the disadvantage that the volume setting cannot reliably be fixed. There is also a risk that an adjustment may involve an "overturning" of the parts so that parts may be damaged.